Kurt Vollmer, Weed Management Specialist
University of Maryland
This is a quick reference chart to check herbicide efficacy for some of the most problematic weeds in corn production: marestail, common ragweed, waterhemp, and Palmer amaranth.
Click here to download a PDF copy.
Kurt Vollmer, Weed Management Specialist
University of Maryland
This is a quick reference chart to check herbicide efficacy for some of the most problematic weeds in soybean production: marestail, common ragweed, waterhemp, and Palmer amaranth.
Click here to download a PDF copy.
Kurt Vollmer, Weed Management Specialist
University of Maryland Extension
Introduction
In Maryland, populations of common ragweed have developed resistance to three sites-of-action. These include Group 9 herbicides (glyphosate), Group 2 herbicides (ALS-inhibiting; Synchrony, Raptor), and Group 14 herbicides (PPO-inhibiting; Flexstar, Valor). These herbicide-resistant populations limit options for effective postemergence control. Previously glyphosate could be used to control common ragweed at various growth stages. However, herbicides that continue to provide postemergence control, such as glufosinate (Liberty) and 2,4-D (Enlist One) are less effective when common ragweed exceeds heights in excessive of 3 inches tall (Figure 1). If preemergence herbicides are ineffective and/or postemergence applications are delayed, common ragweed can quickly exceed this optimal control height. As a result, multiple post-emergence applications may be required to manage this weed.
Objectives
The objective of this study was to evaluate herbicide programs for the management of common ragweed 4 inches tall or greater.
Methods
The study was conducted in a grower field in Snow Hill, MD with a history of herbicide-resistant common ragweed. Enlist E3 soybeans (tolerant to 2,4-D, glyphosate, and glufosinate) were drilled in 15 inch rows on May 19, 2020 at a rate of 52 lb/A. In order to ensure a better stand of common ragweed, additional weed competition was eliminated with a broadcast treatment of glyphosate + Dual II Magnum applied on May 21. Dual II Magnum provides residual control of certain annual grasses and small seeded broadleaf weeds, but does not control common ragweed. Postemergence herbicides were applied on June 9, to 4 to 12 inch tall common ragweed, and a second postemergence application was made 17 days later on June 26. Treatments included Liberty, Enlist One, and Flexstar applied alone or in tank mixes as single or sequential herbicide applications (Table 1).
Table 1. Postemergence herbicide treatments for control of large common ragweed.
| Treatment No. | Application Aa | Rate (pt/A) | Application Bb | Rate (pt/A) | |
| 1 | Liberty | 2.7 | Liberty | 2.7 | |
| 2 | Enlist One | 2 | —- | —- | |
| 3 | Enlist One | 2 | Enlist One | 2 | |
| 4 | Enlist One | 2 | Enlist One + Flexstar | 2 + 1.6 | |
| 5 | Enlist One | 2 | Flexstar | 1.6 | |
| 6 | Enlist One + Flexstar | 2 + 1.6 | Enlist One | 2 | |
| 7 | Flexstar | 1.6 | —- | —- | |
| 8 | Flexstar | 1.6 | Enlist One | 2 |
a.Treatments applied June 9, 2020.
b.Treatments applied June 26, 2020.
Results
Sequential applications were needed to provide at least 95% control of common ragweed (Figure 2). On June 17, prior to the second postemergence application, control of common ragweed with Liberty was less than 70%, and control with Enlist One averaged 56%. On July 22, sequential treatments of Liberty, Enlist, and Flexstar controlled common ragweed better than Enlist One applied once.
Summary
These results emphasize the importance of timely postemergence herbicide applications in controlling common ragweed. Delaying applications increases the likelihood that common ragweed will not be controlled with a single application, and sequential applications will be needed to manage this species effectively. However, additional research is needed to confirm these results and help identify consistent treatments.
In addition, herbicide-tolerant soybeans were needed in order to apply Enlist One and Liberty. Historically, Group 2 herbicides, such as Raptor, and Group 14 herbicides, such as Reflex, have been used to control common ragweed in non-herbicide tolerant soybeans. In this study, Flexstar alone provided good control early on, but common ragweed control declined within a month after application (Figures 2 and 3). Furthermore, in Maryland, Flexstar applications can only be made once every other year, and would not be suitable if sequential applications are required. In order to manage these populations, conventional soybean growers will likely have to adopt or continue to use soybeans tolerant to 2,4-D and/or glufosinate. Several soybean varieties are available that contain tolerance multiple herbicide groups. These not only offer more flexibility for postemergence herbicide programs, but also offer the option of tank-mixing different herbicide groups for improved control. Further research is needed on how this approach can improve control of common ragweed and mitigate further herbicide resistance development.
Acknowledgements
This research was funded by the Maryland Soybean Board.
Ben Beale, Principal Agriculture Agent, St. Mary’s County & Kurt Vollmer, Weed Management Extension Specialist
University of Maryland
Palmer amaranth is now germinating in fields in Southern Maryland (Fig. 1), with the largest plants around 3 inches tall. The cool spring has slowed germination and plant growth this year. At the same time last year, Palmer amaranth plants were over 10 inches tall. Hot weather is returning this week so we can expect to see plants jump in size. In soybeans, now is the critical time to control Palmer amaranth plants before the crop emerges. The optimal size for effective control of this species is 3 to 4 inches. In fields that received a burndown herbicide application in May, expect to see additional plants emerging, often before the soybean crop is planted. Paraquat (Gramoxone® or generic alternatives) is an effective burndown treatment for smaller plants provided it is applied with adequate water to achieve good spray coverage. Other options include products like 2,4-D, saflufenacil (Sharpen®), or dicamba. Each of these products have plant-back restrictions. Glyphosate, while not effective on Palmer amaranth, can still be added to control other weeds. Our recommendation for controlling Palmer amaranth continues to be starting clean, using a residual herbicide with two modes of action at planting, and using a timely postemergence application around 24 days after planting. In our trials, products with flumioxazin (Valor®) including premixes of Fierce® or Fierce XLT® or products with sulfentrazone (Authority®) including premixes Broadaxe®, Authority Elite®, and Authority MTZ® provided the most consistent control. In 2020 trials, we evaluated 15 different herbicide programs in high-pressure sites. Bottom line—preemergence products containing either sulfentrazone or flumioxazin in combination with either metribuzin, pyroxasulfone, or S-metolachlor* provided excellent control for the first three weeks. When used in combination with an effective postemergent product such as Xtendimax® or Reflex® applied 24 days after planting, Palmer amaranth was effectively controlled with 0 plants present 60 days after planting. *S-metolachlor (Dual Magnum®) or dimethenamid (Frontier® or Outlook®) cannot be tank-mixed with Valor® within 14 days of planting soybeans, unless soybeans are planted under no-till or minimum-till on wheat stubble or no-till field corn stubble.
Integrating several strategies into your weed management plan will help avoid resistance and increase control. Effective strategies include crop rotation, tillage, seed bank management, preventing seed movement, use of cover crops, narrow rows (15 inches or less) and managing the crop to promote competitive growth and quick canopy closure.
Common Ragweed and Marestail:
We have already started to receive calls about controlling ALS- and glyphosate-resistant ragweed and marestail in emerged soybeans. Unfortunately, we don’t have any of good options for control once beans are up. The only option for ALS- and glyphosate-resistant ragweed in conventional or Roundup Ready® soybean is fomesafen (Reflex® or Flexstar®).
Marestail plants are beginning to bolt in Southern Maryland (Fig. 2). Most marestail is resistant to glyphosate and ALS products. As with ragweed and Palmer, it is critical to control marestail before planting.
Unfortunately, post emergent PPO products, such as Reflex® or Cobra®, do not control marestail. In ALS- resistant populations, there are few options once beans emerge. Burndown options include glyphosate plus 2,4-D (1 qt/A), glyphosate plus dicamba or glyphosate plus Sharpen®, will effectively control glyphosate- and ALS-resistant populations when they are less than 4 inches tall. Plant back restrictions apply. Liberty® or Gramoxone® are also options for small marestail and do not have any plant back restrictions.
If you planted one of the tolerant herbicide platforms like RR2 Xtend® (dicamba, glyphosate); LibertyLink® (glufosinate); LibertyLink GT27® (glufosinate, glyphosate, HPPD); or Enlist E3® beans (2,4-D, glyphosate, glufosinate) you have an additional option for early control. With the increase in availability of different herbicide tolerance platforms, it is important that producers carefully record where each variety is planted to avoid application mistakes. Glufosinate will kill Xtend® soybeans, and dicamba will kill Enlist, LibertyLink® and Liberty Link GT® beans. A couple of reminders to increase control. Contact products such as Liberty®, Reflex® and Cobra® require good spray coverage to achieve control so use at least 15 gallons of water per acre. Liberty works best when applied in full sunlight. All POST products including Liberty®, dicamba, 2,4-D, Reflex®, Cobra®, and Ultra Blazer® are most effective when weeds are 4 inches tall or less.
More information on management of Palmer amaranth is available here: https://extension.umd.edu/sites/extension.umd.edu/files/_docs/publications/Palmer%20Amaranth%20in%20MD%20FS-1086%20%281%29.pdf
More information on management of Common Ragweed is available here: https://extension.umd.edu/learn/publications/managing-herbicide-resistant-common-ragweed
Kurt Vollmer, Extension Weed Management Specialist
University of Maryland
As the 2020 planting season begins, marestail is bolting and common ragweed has started to emerge (Fig. 1). Populations of marestail in the region are resistant to glyphosate and ALS-herbicides (Group 2, i.e. Classic), and populations of common ragweed are resistant to glyphosate, the ALS-herbicides, and PPO-herbicides (Group 14, i.e. Reflex). Herbicides such as 2,4-D, dicamba, and Gramoxone will provide control of emerged weeds, but they are most effective when applied to weeds less than 4 inches tall. Now is the time to spray these weeds. If you have already burned down your field, scout before planting to make sure these species have not escaped or emerged since your burndown application. If they have, consider adding Gramoxone to your at planting application. Dicamba can be used prior to planting Xtend soybeans or 2,4-D can be used with Enlist crops. Always consult the label for application instructions and approved tank-mixes.
Sarah Hirsh, Agriculture Agent
University of Maryland Extension, Somerset County
In the next couple of weeks, common ragweed will begin to emerge (Figure 1). There are herbicide-resistant populations of common ragweed throughout Maryland. In 2019, common ragweed samples from St. Mary’s, Charles, Somerset, Worcester, and Wicomico counties, MD were tested and found to be resistant to glyphosate (group 9) and cloransulam (Firstrate; group 2 “ALS Inhibitors”) herbicides. Furthermore, three-way resistance to glyphosate, Firstrate, and fomesafen (Reflex, group 14 “PPO Inhibitors”) was found for two eastern shore samples from Dorchester Co, MD and Kent Co, DE. Because common ragweed populations can be resistant to glyphosate and other herbicides, pre-planting and residual herbicide control are particularly important.
What steps can you take now against common ragweed?
More information on managing herbicide-resistant common ragweed in Maryland can be found here: https://extension.umd.edu/sites/extension.umd.edu/files/_docs/publications/Ragweed%20FS474.pdf
Common ragweed can quickly get out of hand and dominate a soybean crop (Figure 2). However, with proactive control and scouting, herbicide-resistant common ragweed can be effectively controlled.
